Think of a platform as a raised stand that something has to be placed on before it can work or be seen. Most things are built to fit one stand and fall off the others. Multi-platform describes the thing that has been built to sit on several stands at once, so the same one thing shows up in several places without being remade.
可以把 platform 想成一個台子：東西要先擺上去，才跑得動、才看得見。多數東西只做得下一個台子，換一個就擺不上。multi-platform 形容的就是那種一次站得上好幾個台子的東西。同一份東西不必重做，卻同時出現在不同地方。
From the word’s history
From multi- plus platform.
由 multi-（多）加上 platform（平台）組成。字面就是「多個台子」，兩個意思都是同一個字面在不同行業裡的落點。
Wiktionary, entry 'multi-platform' (etymology: From multi- + platform); supplied in the evidence packet
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